While the invention is subject to a wide range of applications, it is particularly suited for metal packages to house an electronic device. In particular, the invention relates to epoxy sealed pin grid array-type packages having a metal or metal alloy base component.
Microelectronic devices are typically manufactured from a semiconductor material, such as silicon, germanium or gallium/arsenide. The semiconductor material is fashioned into a die, a generally rectangular structure having circuitry formed on one surface. Along the periphery of that surface are input/output pads to facilitate electrical interconnection to external components.
The semiconductor device is brittle and requires protection from moisture and mechanical damage. This protection is provided by a package. The package also contains an electrically conductive means to transport electrical signals between the semiconductor device and external circuitry.
One package design which minimizes space requirements and provides a high density of interconnections between the electronic device and external circuitry is the pin grid array package. The conventional pin grid array package comprises a multilayer alumina (Al.sub.2 O.sub.3) substrate having conductive circuitry disposed between the layers. The circuitry terminates at a plurality of conductive pads to which terminal pins are brazed. The pins are generally configured into a regular array. U.S. Pat. No. 4,821,151 to Pryor et al discloses a ceramic pin grid array package.
Molded plastic pin grid array packages are also known in the art. In one configuration, disclosed in U.S. Pat. No. 4,688,152 to Chia, a printed circuit wiring board having plated through holes serves as the package base. An integrated circuit device is bonded to one face of the printed circuit board. Lead wires electrically interconnect the integrated circuit device to circuit traces on the board. The circuit traces terminate at conductive rings containing terminal pins which pass through the printed wiring board and exit the opposite surface of the board. The surface containing the integrated circuit, wire bond and circuit traces, is then encapsulated in a molding plastic.
Yet another molded plastic pin grid array package is disclosed in U.S. Pat. No. 4,816,426 to Bridges et al. The patent discloses a circuit tape having terminal pins prebonded to the tape. The pins are electrically interconnected to circuit traces formed on the tape. The assembly is then partially encapsulated within a polymer resin.
Ceramic pin grid array packages have excellent reliability, but are brittle, expensive and are poor conductors of heat. One of the chief advantages of a pin grid array package is the high number of electrical interconnections possible. The more complex the integrated circuit, the more heat generated during operation. If this heat is not removed, the device operating life is decreased. It has been estimated that for every 10.degree. C. increase in operating temperature the effective operating life of the device is decreased by 50 percent.
Plastic pin grid array packages are not brittle and considerably cheaper to manufacture than ceramic pin grid array packages. The thermal performance of a plastic pin grid array package may be improved by molding a metal heat spreader into the body of the package as disclosed in the above cited U.S. Pat. No. 4,816,426. While such a package gives exceptional performance, the large surface area of exposed plastic makes plastic pin grid array packages susceptible to moisture permeation.
One way to achieve the hermeticity and reliability of the ceramic package with the reduced cost and improve thermal performance of the plastic pin grid array package is with a metal pin grid array package. A hermetic metal pin grid array package is disclosed in a Toshiba new product release entitled "METAL PIN GRID ARRAY PACKAGE". The publication discloses a printed wiring board circuit containing a plurality of terminal pins. The pins exit the wiring board and pass through a metal base. An isolation seal electrically isolates the terminal pins from the metal package base.
The composition of the isolation seal is not disclosed. It appears to be a cylindrically-shaped apertured preform of a metal sealing glass composition such as a borosilicate. These preforms are widely used to isolate feed through pins from metal package bases in hybrid packaging. Examples of isolation seals are given in U.S. Pat. No. 4,706,382 to Suppinger et al as well as U.S. Pat. No. 4,716,082 to Ahearn et al.
Disadvantages with the use of isolation seals include cost and difficulty of assembly. Preforms, either glass or plastic, are inserted on each terminal pin. Each terminal pin is then aligned within the metal base prior to formation of the isolation seal. If the seal is glass, it must be carefully selected so that its coefficient of thermal expansion of the glass is close (typically within about 10 percent) to the coefficient of thermal expansion of the package base and the terminal pins. Since the glass is inherently brittle, glass fracture and loss of electrical isolation is a potential problem.